Different kinds of oscillator circuits, i.e. oscillators, are used for a very great number of applications of the electronics and the telecommunications technology. Typical applications of the telecommunications technology are Phase-Locked Loops (PLL), frequency oscillators, modulators, etc.
Oscillator circuits, i.e. oscillators, can be implemented by many different circuit structures. One of them is an astable (free-running) multivibrator.
FIG. 1 shows a conventional emitter-coupled multivibrator circuit. The circuit comprises two transistors Q1 and Q2, between which is provided a positive feedback by connecting each transistor collector via a buffer transistor Q3, Q4 to control a base of the other transistor. In some known solutions Rc1 and Rc2 are replaced by coils. Collectors of Q1 and Q2 are connected via the resistors Rc1 and Rc2, respectively, to one potential of an operating voltage source 1 and emitters are connected via current sources 3 and 4, respectively, to the lower potential of the operating voltage source. Correspondingly, emitters of the buffer transistors Q3 and Q4 are connected via current sources 5 and 6 to the lower potential. Additionally, a capacitance C is connected between the emitters of Q1 and Q2. The positive feedback and series resonance circuits Rc1-C and Rc2-C constituted by the resistors RC1 and RC2 and the capacitance C lead to that the multivibrator output oscillates continuously between two states, after the oscillation once has been trigged. The oscillation frequency is determined by component values of the RC series resonance circuits. The oscillation frequency can be controlled by changing some of these component values, typically the capacitance C.
In the following, the operation of the multivibrator will be examined closer. To begin with, it is assumed that Q1 and Q3 are off (non-conduction state). When Q1 is off, the collector of Q1 and the base of Q2 are generally at the operating voltage potential. Then Q2 is on (conducting state) and its emitter current is I1+I2. Likewise the buffer transistor Q4 is on and feeds base current to Q2. When Q2 is conductive, the current I1 flows from the emitter of Q2 via the capacitance C to the emitter of Q1. Then the current I1 charges/discharges the charge of the capacitance C, whereby the emitter potential of Q1 falls at a predetermined speed until Q1 becomes conductive when the base emitter voltage of Q1 exceeds appr. 0.6 V. When Q1 becomes conductive, its collector voltage begins to fall, due to which the buffer transistor Q3 begins to close. On account of the positive feedback provided via Q4, the base voltage of Q2 falls as well and Q2 closes. Q2 closing makes the collector voltage of Q2 increase, which accelerates the opening of Q3. Q3 opening increases, via positive feedback, the base current of Q1. A higher base current discharges parasitic capacitances of the base circuit of Q1 faster and accelerates thus the opening of Q1. When Q2 is off and Q1 is on, the current I2 flows from the emitter of Q1 via the capacitance C to the emitter of Q2, where the emitter voltage begins to fall until it makes Q2 open and, via Q3, Q1 close again. The speed of such a multivibrator circuit (maximum resonance frequency) depends primarily on the properties of the transistors Q1, Q2, Q3 and Q4. Nowadays, there is a need of ever-increasing-speeds, however.
The minimum operating voltage required by a multivibrator of above type is about 1.5 V, from which at least 0.4 to 0.5 V is used across the current sources 3, 4, 5 and 6. However, especially in electronic equipments using battery power supplies, lower operating voltages would be desired.